Inverse relationship of dimensionality and catalytic activity in CO₂ transformation: A systematic investigation by comparing multidimensional metal-organic frameworks

The correlation between dimensionality and active sites on deciding the catalytic performance of an MOF catalyst in CO₂-epoxide cycloaddition reactions has been studied. Seven In(iii) based MOFs built from carboxylic and N-donor ligands possessing different dimensionalities and distinct coordination environments were chosen as solid acid catalysts for this study. The origin of the catalytic activity of an In³⁺/TBAB bifunctional system in a CO₂-PO reaction was studied in detail by performing density functional theory (DFT) calculations at the M06/LACVP∗∗++ level. The energy barrier of the propylene oxide ring opening in the presence of In³⁺/Br^- is 11.5 kcal mol^-1, which is significantly lower than those of un-catalyzed (55-63 kcal mol^-1) and Br^--catalyzed (19.5 kcal mol^-1) reactions, which confirms the importance of the In³⁺/Br^- binary catalytic system in the CO₂-epoxide cycloaddition reactions. The one-dimensional (1D) MOF with unsaturated metal centers exhibited higher catalytic activity (PO conversion: 91%, temperature: 50 °C, and time: 12 h) than the two- and three-dimensional MOFs. The roles of dimensionality and unsaturated metal centers in cycloaddition reactions were explained on the basis of the results of activity testing and structural investigations. In addition, a plausible reaction mechanism for the catalytic activity of the 1D MOF was proposed with reference to our structure-density functional theory correlations.